Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Utility Patent
1998-11-25
2001-01-02
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C600S422000
Utility Patent
active
06169401
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical diagnostic imaging and will be described with reference thereto. It is to be appreciated, however, that the invention may find further application in quality control inspections, spectroscopy, and the like.
In early Magnetic Resonance Image (MRI) scanners, resonance signals were received by placing linearly polarized radio frequency (RF) coils adjacent to the surface of a patient. The linearly polarized coils received only one component of magnetic resonance signals, commonly, the component in either the horizontal or vertical direction. By contrast, the magnetic resonance signals emanating from the subject are more accurately defined by a vector which rotates in a plane, i.e. has two orthogonal components. Thus, the linearly polarized coil only received one of the two orthogonal components.
A quadrature coil, which has a circularly polarized magnetic field, receives orthogonal components of the rotating vector. A quadrature coil can support both the horizontal and vertical made current distributions. Thus, the quadrature coil extracts twice the signal power from the rotating vector than does the linearly polarized coil with the same noise. This results in a signal-to-noise ratio which is greater by the square root of two or about 41%. However, prior art quadrature coils are commonly volume coils rather than surface coils. For example, some volume quadrature coils have included two saddle coils which were rotated 90° relative to each other. The portion of the patient to be imaged is disposed within the volume defined in the interior of the saddle coils. Analogously, other coils have been utilized which define a volume around the circularly polarized region.
Others have recognized the desirability of a quadrature surface or flat coil. See, for example, U.S. Pat. No. 4,918,388, issued Apr. 17, 1992, Mehdizadeh, et al. The Mehdizadeh surface coil includes a separate loop and butterfly coil arrangement disposed on opposite sides of a dielectric sheet. The two circuits are mutually decoupled from each other for the respective resonance modes of interest.
Integrated coil assemblies have been proposed. H. Liu, et al. reported a quadrature surface coil in the Proceedings of the International Society For Magnetic Resonance In Medicine, 1997, page 1493. That coil consisted of five uniformly spaced capacitive axial elements sandwiched between two conductive elements. Liu disclosed a symmetric lowpass coil. That is, the Liu coil detects both even currents and odd currents in the lower resonant frequencies. Modeling of such structures has shown undesirable localized electric field effects leading to coil tuning instability. This instability is due to the small capacitance values on each axial element required because of the lowpass configuration. Moreover, many capacitive elements in series were required in modeling, which is not ideal from a manufacturing view.
Another integrated coil is described by Boskamp et al. in the Proceedings of the International Society of Magnetic Resonance in medicine, 1998, page 2024. The Boskamp coil has a symmetrical degenerate bandpass configuration in which the two lowest modes occur at the same frequency. Boskamp's device is a bandpass coil, that, while more stable than a lowpass, is still not optimum. Moreover, tuning a coil according to Boskamp requires varying the capacitance of each capacitor on either the axial or horizontal element. This necessitates a time consuming, expensive tuning process depending on the desired frequency match.
Additionally, both the Liu and Boskamp devices may require use of a 180° phase shift cable (on the order of 1 meter in length) as well as inductors to match the frequency of the two desired modes.
The present invention contemplates a new, improved open quadrature highpass RF surface coil which overcomes the above difficulties and others.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, a magnetic resonance apparatus includes a main magnetic field generator for providing a magnetic field through an examination region. A magnetic resonance excitor is also included for exciting a nuclei of the subject in the examination region to generate magnetic resonance signals. A first radio frequency coil detects the magnetic resonance signals from the resonating nuclei of the subject. The coil includes N parallel legs, where N is an odd integer equal to or greater than 5. Electrically conductive sides interconnect ends of the legs while a plurality of side capacitive elements are electrically connected in series along each of the sides. A central one of the N legs, having at least one central leg capacitive element is also included. The side and central leg capacitive elements are symmetric about a midpoint of the central leg such that the coil supports an even mode which is sensitive to a resonance signal component in the plane of the coil and an odd mode which is sensitive to resonance signals in a plane orthogonal to the plane of the coil.
In accordance with another aspect of the present invention, the radio frequency coil further includes a sampling port electrically in parallel to at least one of the central leg capacitive elements through which the even mode is sampled.
In accordance with another aspect of the present invention, the radio frequency coil includes a sampling port electrically connected parallel to two of the side capacitive elements symmetrically about the central leg through which the odd mode is sampled.
In accordance with another aspect of the present invention, the radio frequency coil further includes the capacitive elements being selected to allow magnetic resonance signals above a pre-selected frequency to pass substantially unimpeded, while frequencies below the preselected frequency are substantially impeded such that the radio frequency coil operates in a highpass mode.
In another aspect of the present invention, a second radio frequency coil is placed partially overlapping the first radio frequency coil such that the mutual inductance therebetween is minimized.
In accordance with the present invention, a quadrature highpass ladder coil for magnetic resonance imaging includes a central leg having one or more of central leg capacitive elements. A plurality of legs without capacitive elements are included parallel to and symmetrically from each side of the central leg. Also included, are a plurality of side capacitive elements, each being electrically connected between a first adjacent end of each of the plurality of legs and between second adjacent ends of each of the plurality of legs. The legs and side capacitive elements support two frequency modes, an even mode sensitive to radio frequency fields in a plane parallel to that of the legs and an odd mode sensitive to radio frequency fields orthogonal to the plane of the legs.
In accordance with another aspect of the present invention, the side and central leg capacitive elements are selected such that the odd and even modes have peak sensitivity to a common radio frequency.
One advantage of the present invention resides in an open RF coil structure which is easy to manufacture.
Another advantage of the present invention resides in an optimized current distribution that supports both even and odd current distributions along the axial conductors.
Another advantage of the present invention resides in a highpass implementation, reducing the localized electric field effects that can lead to coil tuning instability.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiment.
REFERENCES:
patent: 4816765 (1989-03-01), Boskamp
patent: 4918388 (1990-04-01), Mehdizadeh et al.
patent: 5030915 (1991-07-01), Boskamp
patent: 5309104 (1994-05-01), Frederick
patent: 5510714 (1996-04-01), Takahashi et al.
patent: 5559434 (1996-09-01), Takahashi et al.
patent: 5578925 (1996-11-01), Molyne
Burl Michael
Fujita Hiroyuki
Morich Michael A.
Arana Louis
Fay Sharpe Fagan Minnich & McKee LLP
Picker International Inc.
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